Patent Literature 1 discloses an example of a conventional synchronous motor control device including a current detection unit that detects a current flowing into a synchronous motor, a current coordinate transformation unit that transforms coordinates of the current detected by the synchronous motor into γ-δ axis assumed on a rotor, a compensation term calculation unit that calculates a compensation term on the basis of a correction current command and the coordinate-transformed current, and a voltage command calculation unit that calculates a γ-δ voltage command on the basis of a voltage equation of the synchronous motor in a steady state and the compensation term, where the device feeds a direct current in accordance with the position of a magnetic pole being detected during start-up and estimates a primary resistance of the synchronous motor. According to this technique, a calculation load can be reduced by calculating the voltage command value with use of the voltage equation, while at the same time the primary resistance of the synchronous motor is estimated during start-up to be able to follow a change in coil resistance even under the environment in which there is a large change in temperature as with the synchronous motor mounted in a compressor.